By some measures, technology stocks are doing better now than any time since the dot-com boom.

Alphabet and Facebook are no exception. Both stocks hit all-time highs this summer, and both have risen more than 30 percent over the past year. It's no surprise, then, that 86 percent of analysts listed by FactSet have a buy or overweight rating on Alphabet's Class A shares, while 89 percent have a buy or overweight rating on Facebook.

"I cover the advertising-centered names, specifically, so I would also include Adobe and Salesforce among the companies that I look at," Wieser said. "Unfortunately, there's just been too much of a runup, so I find it hard to find value. Frankly, I have one company out of 17 I cover that I have a buy on, and that's Fox. Everything else is either a hold or a sell at this point."

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Part of the issue may be that passive or generalist investors, who don't know the technology space, are moving money toward the high-growth sector, Wieser said. The S&P technology grouping is up nearly 23 percent so far this year, posting far bigger gains than any other sector.

"I think there is a bit of a crowded trade, yes. But I think even among those who are, let's say, specialists in the sector, who understand it — or who claim to understand it — I think there's a lot of willful optimism," Wieser said.

In the advertising space, for instance, if analysts are assuming that advertising spending will sustain its growth rate for the next five years, simple math would indicate that the entire advertising market would be held by Facebook and Google, Wieser said.

"That's not realistic," Wieser said. "Investors are looking at numbers unrealistically. There's an assumption that they can perhaps capture all of marketing spend, not just advertising spend, and therefore there's much more room to grow."

He said that as more companies look to keep eyeballs glued to their advertisements, they'll have to offer more and better content at the same price as competitors. That means more expenses for the same amount of profits, which could hamper long-term growth.

It's the same kind of argument made by critics of the "attention economy," such as Tristan Harris, who have argued that technology companies are trying to get people hooked, rather than build products that consumers want.

While Wieser is a "big fan" of Facebook as a company, especially compared with Snap, he said Alphabet has the most downside protection if the advertising technology market were to grow less than expected.

"No one says the value of a market has to go up, right? I mean, in terms of the consumer spending," Wieser said. "It's not a given that that happens, necessarily. I think the reality is that consumers don't need to spend more money to be fully entertained through their video services."